1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an X-ray detector for deriving an optical image from an X-ray image, an image pick-up apparatus for picking up the optical image, which image pick-up apparatus includes a plurality of image sensors for deriving separate electronic sub-image signals from the optical image, and a subtraction unit for deriving a difference signal from the electronic sub-image signals.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from German Patent DE 11 23 410.
The known X-ray examination apparatus includes two television cameras which simultaneously pick up the optical image. One of the television cameras is set out-of-focus so that it picks up only coarse structures in the optical image. The other television camera is accurately focused so that it accurately picks up coarse structures as well as fine details in the optical image. Each of the television cameras delivers a video signal, one video signal representing the coarse structures and the small details whereas the other video signal represents the coarse structures only. The video signals are subtracted from one another and the resultant difference signal is applied to a monitor. The difference signal thus represents mainly the small details and these details are displayed with high contrast on the monitor. Therefore, the known X-ray examination apparatus is suitable for high-contrast reproduction of image information relating to smalls in the X-ray image. Because the video signals relate to different components of simultaneous image information, the known X-ray examination apparatus is not suitable for forming a difference image representing changes in the optical image. Changes occur, for example when a contrast medium is injected into a patient to be examined.